Digital radio receiver

ABSTRACT

A receiving unit ( 2 ) receives a modulated signal resulting from modulating a carrier signal with a digital encoded train resulting from error-correction encoding a digital signal train. A demodulating unit ( 4 ) demodulates the digital encoded train from the received modulated signal. A decoding unit ( 8 ) decodes the digital signal train from the demodulated digital encoded train. The receiving unit ( 2 ) outputs a received-signal strength indicative signal indicative of the received-signal strength of the modulated signal. During demodulating, a bit error rate computing unit ( 18 ) of the decoding unit ( 8 ) computes a bit error rate. The received-signal strength indicative signal and the bit error rate are inputted to a control unit ( 14 ), and, on the basis of these inputs, it is judged which one of the receiving conditions defined by received-signal strength indicative signals and bit error rates, the current receiving condition corresponds to.

TECHNICAL FIELD

This invention relates to a receiver for receiving a modulated signalcomprising a carrier signal modulated with a digital signal, and, moreparticularly, to a digital wireless receiver which can give indicationof a signal receiving condition.

BACKGROUND OF THE INVENTION

Digital wireless transmission is sometimes used to transmit analog audiofrequency signals and analog video signals. In digital wirelesstransmission, a digital signal train produced by digitizing an audiofrequency signal or video signal is error-correction encoded to therebydevelop a digital encoded train in a transmitter. With this digitalencoded train, a carrier signal, which is a high-frequency signal, ismodulated to provide a modulated signal. The modulated signal istransmitted through a transmitting antenna. At a receiver side, themodulated signal received by a receiving antenna is demodulated toprovide the digital encoded train, from which the digital signal trainis decoded. The decoded, digital signal train is converted back into theanalog audio frequency signal or analog video signal in adigital-to-analog converter.

In wireless transmission where a transmitter and a receiver are used, acommunication area in which proper wireless transmission is available isdetermined by transmission power of the transmitter and sensitivity ofthe receiver. Therefore, a receiver in such communication area cannotsatisfactorily receive well a modulated signal from a transmitteroutside the communication area. Because radio frequencies available forwireless microphones, when used as such transmitter, are limited, thefrequency to be used for the wireless microphone must be selected fromsuch frequencies. When the wireless microphone is being operated at onefrequency, there may be an apparatus operating at the same frequency orat a frequency which interferes with the frequency of the wirelessmicrophone. In such a case, a receiver for that wireless microphonecannot properly receive the modulated signal from the wirelessmicrophone.

If the transmitter transmits the modulated signal from outside thecommunication area, or if the receiver is receiving the modulated signaltransmitted from the transmitter within the communication area, but itis disturbed by other apparatus, the decoded output from the receivermay contain interruption in sound or picture or contain noise. However,it is not possible to know on the spot whether such interruption insound or picture or noise is due to transmission from outside thecommunication area or due to disturbance by other apparatuses. In orderto determine it, it is necessary to use a measuring device, e.g. a fieldstrength measuring device, to monitor the reception field strength andknow the signal receiving condition.

For example, a cellular phone is provided with a function to indicatewhen it is in an area outside the service area. Accordingly, if onecannot talk through the cellular phone, he or she can know it is becausehe or she is outside the service area, when he or she sees the screen onthe cellular phone. Also, the cellular phone system employs amulti-channel access system, in which any one of a multiple frequenciescan be used. Accordingly, if the reception by the cellular phone isdisturbed by other apparatus, the frequency used by the cellular phoneis automatically changed. Thus, it is less likely that cellular phonesbecome to fail to communicate due to disturbance from other apparatus.In contrast, systems, such as a wireless microphone system, whichneither change automatically the operating frequency nor have anout-of-communication-area indicating function, it is not possible toknow the reception condition or to automatically improve thecommunication condition.

An object of the present invention is to provide a digital wirelessreceiver which can indicate the reception condition by taking advantageof properties of digital wireless transmission.

DESCRIPTION OF THE INVENTION

A digital wireless receiver according to the present invention includessignal receiving means. The receiving means is adapted to receive amodulated signal comprising a carrier signal modulated with a digitalencoded train produced by error-correction encoding a digital signaltrain. Each of the digital signals forming the digital encoded trainincludes a plurality of bits. Needless to say, each of digital encodedsignals forming the digital encoded train is formed of plural bits.Demodulating means demodulates the received modulated signal to recoverthe digital encoded train. Taking advantage of the digital encodedtrain's being error-correction encoded, the digital signal train isrecovered from the demodulated digital encoded train in the demodulatingmeans. The digital wireless receiver according to the present inventionis provided with received-signal strength detecting means which developsa received-signal strength indicative signal indicative of the strengthof the received modulated signal. Bit error rate computing meanscomputes a bit error rate when the modulated signal is being demodulatedin the demodulating means. Decoding means performs error correctionduring the demodulation, taking advantage of error correction encodingprovided for the digital encoded train. The bit error rate is determinedby counting the number of bit errors found in the error correctionprocessing, and computing a ratio of the number of the bit errors to thenumber of bits of the digital encoded train read in in a given timeperiod. The received-signal strength indicative signal and the bit errorrate are inputted to judging means, which makes judgment, based on theseinputs, which one of a plurality of reception conditions defined byreceived-signal strength indicative signals and bit error rates, thecurrent reception condition corresponds to. The plural receptionconditions may include a satisfactory reception condition, anout-of-communication-area communication condition, anon-the-border-of-communication-area communication condition, adisturbing signal reception condition, a receiver failure condition,etc.

When only the received-signal strength is used to determine thereception condition of the receiver, the cause for improper signalreception cannot be known because the received-signal strength increaseseven when a disturbing signal is received. For digital wirelessreceivers, a disturbing audio signal wave cannot be heard as an ultimatesound signal but is heard as noise. In such cases, it is particularlyhard to determine the cause. On the other hand, if only the bit errorrate is used to know the reception condition, it is also hard todetermine the cause for improper reception condition because the biterror rate increases not only when the received-signal strength issmall, but also when the signal of large strength is interfered with adisturbing signal.

Because the digital wireless receiver according to the present inventionis arranged to determine the cause of unsatisfactory reception, based onthe received-signal strength and the bit error rate, it can correctlyknow the reception condition. For example, when the received-signalstrength is large and the bit error rate is also large, the receiver canbe judged to be in a reception condition where a disturbing wave giveslarge influences. If the received-signal strength is small and the biterror rate is large, the receiver can be judged to be in a receptioncondition where the receiver is in an out-of-communication area. If thereceived-signal strength and the bit error rate are ones between therespective values in the above-described two reception conditions, itmay be judged that the receiver is close to the border of thecommunication area. When the received-signal strength is large with asmall bit error rate, the receiver can be judged to be in a satisfactorysignal reception condition. If the bit error rate is small with a smallreceived-signal strength, the receiver may be judged to have failedsince such situation usually does not occur. As described, by the use ofboth of the bit error rate and the received-signal strength, it ispossible to correctly grasp in what signal reception condition thereceiver is.

The judging means may contain a reference table having stored thereinvarious signal reception conditions indexed to various received-signalstrengths and various bit error rates. Determining means determines areception condition from the reference table, which corresponds to thecomputed bit error rate and the received-signal strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram of a receiver for a digital wirelessmicrophone, according to one embodiment of the present invention.

FIG. 2 shows the relationship of a bit error rate to a reception leveland the relationship of a received-signal strength indicative signal tothe reception level in the receiver shown in FIG. 1.

FIG. 3 shows reception conditions determined by the received-signalstrength indicative signal and the bit error rate in the receiver ofFIG. 1.

FIG. 4 shows the relationship between the bit error rate and thereceived-signal strength indicative signal in the receiver of FIG. 1.

FIG. 5 is a diagram corresponding to that of FIG. 4, with a failurecondition and a disturbing signal reception condition additionallyshown.

FIG. 6 is a diagram corresponding to that of FIG. 5, with a satisfactoryreception condition additionally shown.

FIG. 7 is a flow chart of the operation of the receiver of FIG. 1.

BEST MODE FOR EMBODYING THE INVENTION

A digital wireless receiver according to one embodiment of the presentinvention is a receiver for, for example, a digital wireless microphone.As shown in FIG. 1, the receiver has a receiving antenna 2, whichreceives a modulated signal, e.g. a transmitted signal, from atransmitter, e.g. a wireless microphone.

In a digital wireless microphone system, an audio frequency signal, e.g.a sound signal, collected by sound collecting means, e.g. a microphone,is applied to an A/D converter and sampled at a predetermined samplingfrequency to thereby convert it into a digital signal including apredetermined number of bits. Thus, the A/D converter outputs a digitalsignal at the predetermined sampling frequency. These digital signalsform a digital signal train. The digital signal train iserror-correcting encoded, which enables error correction, and whereby adigital encoded train is formed. Any one of known error correctingencoding techniques can be employed. A carrier signal, which is a highfrequency signal at a predetermined frequency, is modulated with thedigital encoded train. The modulated signal is transmitted via atransmitting antenna of the wireless microphone.

The modulated signal received at a receiving antenna 2 of the digitalwireless microphone receiver is applied to receiving means, e.g. areceiving section 4, where the modulated signal is converted to apredetermined intermediate frequency signal and outputted.

The intermediate frequency signal is applied to demodulating means, e.g.a demodulating section 6, where it is demodulated, and the digitalencoded train and a clock recovered from the digital encoded train areprovided. The digital encoded train and the clock are coupled todecoding means, e.g. a decoding section 8, where the digital encodedtrain is decoded into the digital signal train. The digital signal trainis, then, subjected to digital-to-analog conversion, and, the resultantsignal is applied to a loudspeaker 10.

The receiving section 4 is provided with received-signal strengthdetecting means, e.g. a received-signal strength indicative (RSSI)signal generating unit 4 a, which provides a received-signal strengthindicative signal. The received-signal strength indicative signal isgenerally proportional to the reception level of the modulated signal asreceived at the receiving antenna 2, as shown in FIG. 2. For example,the received-signal strength indicative signal is proportional to thereception level within a range of from about 10 dBμV to about 60 dBμV.The received-signal strength indicative signal is converted to a digitalreceived-signal strength indicative signal in an A/D converter 12 andapplied to judging means, e.g. a control section 14.

The decoding section 8 includes an error correcting unit 16 forcorrecting an error added to a signal during the transmission from thetransmitter to the receiver, where error correction is achieved when thedigital encoded train is decoded into the digital signal train. Theerror correction is done in accordance with the error correctionencoding provided on the digital wireless microphone side. The errorcorrecting unit 16 determines whether or not each of the digital signalsforming the respective digital signal train contains error, and, ifpresent, corrects such error.

The error correcting unit 16 includes also bit error rate computingmeans, e.g. a bit error rate computing unit 18, which counts errorsdetected during the error correcting processing and computes a bit errorrate (BER) representing the number of error bits contained in thedigital signal trains read in in a predetermined unit time to the totalnumber of bits in the digital signal trains. As shown in FIG. 2, forexample, the bit error rate is large, about 1e-1 (which means that oneof ten bits is an error bit), when the reception level is low, e.g.about 10 dBμV. As the reception level becomes higher, e.g. about 40dBμV, the number of error bits decreases to about 1e-5 (which means thatone of one hundred thousand (100,000) bits is an error bit). The biterror rate is also applied to the control unit 14.

The control unit 14 may be formed of a CPU, for example, and includesreference tables 20. Using the reference tables 20, the control unit 14determines which one of the reception conditions shown, for example, inFIG. 3 the digital wireless microphone receiver is in. The receptionconditions may be an equipment (receiver) failure condition indicated bya letter “a”, an out-of-communication-area communication conditionindicated by a letter “b”, an on-the-border-of-communication-areacommunication condition indicated by a letter “c”, a disturbing signalreception condition indicated by a letter “d”, and a satisfactoryreception condition indicated by a letter “e”.

The determination of the reception condition may be done in thefollowing manner, for example. In FIG. 2, the received-signal strengthindicative signal is prepared based on the reception level, and,therefore, is determined uniquely, so that it can be determined withconsistency. On the other hand, although the bit error rate iscorrelated to some extent with the reception level as described above,it is not uniquely determined and varies to some extent. The bit errorrates shown in FIG. 2 are average or normal values. In other words, thebit errors occur randomly. Accordingly, if the time for measuring thebit error rate is insufficient, the measured bit error rates will havevarying values. By measuring the bit error rates for a sufficient timeperiod, they converge on a fixed value. FIG. 2 shows such convergencevalues as average values.

What the control unit 14 bases its judgment on are the received-signalstrength indicative signal and the bit error rate, and, therefore, therelationship, derived from FIG. 2, between the received-signal strengthindicative signal and the bit error rate can be expressed by a curveshown in FIG. 4. The curve represents a relationship such that, forexample, as the received-signal strength indicative signal increases,the bit error rate decreases in a range of from a first condition inwhich the received-signal strength indicative signal has a predeterminedfirst received-signal strength indicative value and the bit error ratehas a predetermined first bit error rate value, to a second conditionwhere the received-signal strength indicative signal has a predeterminedsecond received-signal strength indicative value larger than the firstreceived-signal strength indicative value and the bit error rate has apredetermined second bit error rate smaller than the first bit errorrate. Since, as described above, the bit error rate exhibits variety invalues, a marginal range determined, taking into account an allowablewidth for variations, is set on opposite sides of the curve, as shown inFIG. 5. In other words, the bit error rates on the curve are translatedby a predetermined value in both the increasing and decreasingdirections of the received-signal strength indicative signal value,whereby two curves are drawn. The marginal range is a range definedbetween the two curves. The reception condition in which thereceived-signal strength indicative signal and the bit error rate arewithin the marginal range is a reception condition under a normalenvironment, i.e. the normal reception condition. When thereceived-signal strength indicative signal and the bit error rate are inthis normal reception condition, there are substantially no breakings insound and picture. The reception condition and also the bit error ratemay change in the normal reception condition, depending on thetransmission level and the distance of the receiver from thetransmitter.

Normally, it does not occur that the bit error rate is below the biterror rate range of the normal reception condition when thereceived-signal strength indicative signal is below the received-signalstrength indicative signal range of the normal reception condition.Accordingly, if a situation occurs in which the received-signal strengthindicative signal is below the received-signal strength indicativesignal range of the normal reception condition and the bit error rate isbelow the bit error rate range of the normal reception condition, suchsituation is judged to be failure of the receiver.

It may be considered that the reception is affected by a disturbing wavewhen the received-signal strength indicative signal is above thereceived-signal strength indicative signal range of the normal receptioncondition and the bit error rate is above the normal reception signalstrength indicative signal range. Then, it is judged to be a disturbingsignal reception condition.

When the received-signal strength indicative signal is within thereceived-signal strength indicative signal range of the normal receptioncondition with the bit error rate being within the bit error rate rangeof the normal reception condition, there is no failure of the receiverand a signal can be received without being affected by a disturbingwave. Now, the normal reception condition where the bit error rate issmaller than a given value, e.g. 1e-5, is considered as a good receptioncondition as indicated by letters “e1” in FIG. 6. In the disturbing wavereceiving range, if the bit error rate is within a range of less than1e-5, as indicated by letters “e2” in FIG. 6, no practical problems willbe raised, in spite of influence of such disturbing wave, since thereare few bit errors occurring. The combination of the regions with theletters “e1” and “e2” attached is the satisfactory reception conditionwith the letter “e” attached thereto shown in FIG. 3.

In the normal reception range, if the bit error rate is above apredetermined value, e.g. equal to or larger than 1e-5, the receptionlevel is low and the bit error rate is high. Therefore, such region isconsidered as a region near the border of the communication area, asindicated in FIG. 6. In the region near the border of the communicationarea, many bit errors occur, but communications are possible.Accordingly, only if the perfect error correction is provided, therewill be no noise. However, slight environmental changes, e.g. a slightchange in distance between the transmitter and the receiver, may causenoise or lower the reception level, so that communication may becomeimpossible.

A receiver is arranged to judge that it is in an unsatisfactoryreception condition when the reception level becomes below apredetermined value, and a muting circuit in a receiving circuit oroutput circuit thereof is activated, to thereby prevent noise from beingdeveloped. Then, a region in the on-the-border-of-communication-areawhere the reception level is below the level necessitating the muting,i.e. where the received-signal strength indicative signal is below 0.8V, for example, that corresponds to the reception level necessitatingthe muting, is judged to be the out-of-communication area, as shown inFIG. 3.

The reference tables 20 are used in making such judgment. The referencetables 20 are provided for respective ones of the bit error rates, andstore therein the reception conditions corresponding to the respectivevalues of the received-signal strength indicative signals at therespective bit error rates. The reference table 20 to be referenced tofor the bit error rate is determined, the received-signal strengthindicative signal is inputted to the determined reference table, and thereception condition for the received-signal strength indicative signalis read out.

Specifically, a received-signal strength indicative signal is measuredplural times during one measurement of a bit error rate, and the pluralmeasurements of the received-signal strength indicative signal isaveraged. The average received-signal strength indicative signal and themeasured bit error rate are inputted to the reference table 20 todetermine the reception condition. For higher precision, thedetermination of reception condition is carried out plural times, andthe result is displayed as the judgment of the reception condition, on adisplay 22 associated with the control unit 14. If the result indicatesthat the individual reception conditions occur at the same frequency, aresult of judgment is “unstable reception”.

FIG. 7 is a flow chart for judging the reception condition by means ofthe control unit 14. In this judgment processing, first a predeterminednumber, e.g. X, is set as a count R in a counter which counts the numberof times the determination of the reception condition is to be done(Step S2). Next, a count in a counter N is set to zero (0), whichcounter counts the number of times the received-signal strengthindicative signal has been measured for one determination of thereception condition (Step S4). This completes the initial setting.

Next, the measurement of the bit error rate is started (Step S6). Then,whether the bit error rate is being measured is judged (Step S8). If themeasurement is being made, the received-signal strength indicativesignal is converted to a digital received-signal strength indicativesignal in the A/D converter 12 (Step S10), and the resulting data isstored in a buffer provided for the control unit 14 (Step S12). Then,the count in the counter N is incremented by one (1) (Step S14). Theprocessing returns to Step S8. This means that while the bit error rateis being measured, the received-signal strength is also measured.

When the bit error rate measurement is completed (i.e. when the answerto the query in Step S8 is NO), the bit error rate is temporarily stored(Step S16), and the N digital received-signal strength indicativesignals stored in the buffer are averaged (Step S18).

Then, the temporarily stored bit error rate is used to determine whichone of the reference tables 20, defining the relation between receptionconditions and received-signal strength indicative signals, should bereferenced to (Step S20). Next, it is determined which one of thereceived-signal strength indicative signals in the determined referencetable 20 the averaged received-signal strength indicative signalcorresponds to (Step S22) to determine the reception condition, and thedetermined reception condition is stored in a result buffer (Step S24).The count in the counter R is decremented by one (1) (Step S26), and ajudgment as to whether the count in the counter R has become zero (0) orless (Step S28). The processing of from Step S4 to Step S28 is repeateduntil the judgment in Step S28 becomes YES. When the answer to the queryin Step S28 becomes YES, the judgments for the X reception conditionsare collected in the result buffer.

When the answer to the query in Step S28 becomes YES, the receptionconditions in the result buffer are processed together (Step S30), andwhether any result has been obtained or not is judged (Step S32). Let itbe assumed that it has been determined that, for example, for X=10, thesatisfactory reception has occurred seven times, theon-the-border-of-communication-area reception has occurred once, theout-of-communication area reception has occurred once, a disturbing wavereception has occurred once, and the device failure has occurred zerotimes. The most frequently occurring reception, i.e. the satisfactoryreception is adopted as the ultimate judgment. If no substantialdifference is present in number of occurrences among the respectivereception conditions, the ultimate judgment is “unstable reception”(Step S34). The obtained ultimate judgment is displayed on the display22.

When reception is degraded, a user can see the display on the display 22to know which is the cause for such reception degradation, transmissionfrom out of the communication area, transmission from a place on theborder of the communication area, failure of the receiver, or disturbingwaves, and, therefore, the user can take an action against itimmediately.

The present invention has been described to be embodied in a receiverfor a digital wireless microphone, but it is not limited to suchreceiver. Rather, the present invention can be used for variousreceivers which are adapted to receive digital modulated signals.According to the described embodiment, a reference table to be used isdetermined in accordance with a bit error rate, and a receptioncondition is determined from the determined reference table inaccordance with a received-signal strength indicative signal. However, areference table, which, in this case, stores therein a relationshipbetween bit error rates and reception conditions, may be determined inaccordance with a received-signal strength indicative signal, and areception condition is determined from the reference table in accordancewith a bit error rate. Also, according to the described embodiment, areceived-signal strength indicative signal is converted into a digitalreceived-signal strength indicative signal in the A/D converter 12 forapplication to the control unit 14. However, without using the A/Dconverter 12, a received-signal strength indicative signal, which is ananalog signal, may be compared, in judging means, with a plurality ofthreshold values determined in accordance with bit error rates todetermine a reception condition. This latter arrangement enablessimplification of circuitry, high-speed operation, and power saving.

1. A digital wireless receiver comprising: receiving means for receivinga modulated signal including a carrier signal modulated with a digitalencoded train, said digital coded train comprising an error-correctionencoded digital signal train; demodulating means for demodulating thereceived modulated signal to provide said digital encoded train;decoding means for decoding said digital encoded train to provide saiddigital signal train received-signal strength detecting means fordeveloping a received-signal strength indicative signal indicative of areceived-signal strength of said modulated signal; bit error ratecomputing means for computing a bit error rate during said decoding; andjudging means to which said received-signal strength indicative signaland said bit error rate are inputted, said judging means using saidreceived-signal strength indicative signal and said bit error rate todetermine which one of a plurality of signal receiving conditionsdefined by received-signal strength indicative signals and bit errorrates the receiving condition corresponds to.
 2. The digital wirelessreceiver according to claim 1 wherein said judging means includes areference table having various signal receiving conditions storedtherein, said various signal receiving conditions being indexed tovarious received-signal strengths and bit errors, and determining meansfor determining, from said reference table, a signal receiving conditioncorresponding to said computed bit error rate and said received-signalstrength indicative signal correspond to.